The axis can be driven by means of a rotating drive unit and/or by means of a linear drive unit. A torque acting on the drive unit or a force corresponding to said torque is measured along a predetermined movement path of the axis.
Methods are generally known from the prior art for monitoring the function of a machine, such as a machine tool for example. The monitoring is typically undertaken on the basis of operating parameters determined or suitable characteristic values. With machines featuring one or more axes characteristic values for describing the movements of the axes are especially known. The characteristic values are for example a minimum, a maximum or an mean torque. These rotational characteristic values are advantageous if the respective machine axis is driven by a rotating drive unit. Such a drive unit mostly features a (rotating) electric motor, which moves the corresponding axis of the machine in a linear manner via a spindle drive. The drive unit can additionally include a converter which then feeds the electric motor. Typically the rotational characteristic values are determined in a synchronization test. This is done by measuring the torque acting on the drive unit torque at a predetermined constant speed of movement. The characteristic values determined in this way can then be monitored to check whether they are exceeding or falling below a comparison or limit value.
In the case of a horizontally arranged axis, the torque measured in the synchronization test then corresponds to a friction moment which must be exerted by the drive unit to compensate for the friction of the axis. If for example a measured mean torque, which in the present case corresponds to an mean frictional moment, exceeds a comparison value, this can indicate increased friction and consequently wear or a lack of lubrication of axis components. An appropriate automated warning message can then be output to a service engineer.
If the drive unit of the axis is a linear drive with a linear motor, then the electrical energy fed into the linear motor direct can be converted directly into a linear movement of the axis. In this case translational characteristic values are more advantageous to describe the axis movement. Such characteristic values are—like the rotational typical characteristic values—a minimum, a maximum or a mean force.
In the case of a horizontally arranged axis the direction in which the axis torque or the axis force is measured, i.e. in the forward direction or the reverse direction, is basically of no significance, since no energy-conserving forces or moments act via the axis on the drive unit. However directional interference variables, such as sticking points in the guide track of the axis can make a separate directionally-dependent determination of the characteristic values and thus a separate monitoring necessary.
If on the other hand the respective axis is not aligned horizontally, for example vertically or sloping at an angle, the weight force acts as a path-independent force in only one direction. Such axes are also referred to as “suspended” axes. This force is thus higher to move the axis in the one direction and lower in the opposite direction. In other words a higher torque is required if the axis is to be moved to a higher energetic level. If the reverse applies the required torque is smaller. It can also be negative depending on the incline of the axis. In this case the drive slows the axis down.
The respective axis can also be moved by means of two or more drives, with the controlled drives being coupled together. In such cases reference is made to coupled drives. The situation can occur here in which one or more drives partly work “against each other”, if for example the zero or the reference points in the measurement systems of the drives are different from one another. In such cases the drives can operate against each other over the entire movement path or only partially as well as with positionally-dependent different forces. For example two drives can be present which drive the axis in the same direction of movement. If the second drive in the comparison exhibits a slightly lower moment or a slightly lower force by comparison with the first drive, the second drive has a braking or driving effect on the first drive. It now “appears” for the overall drive as if a path-independent weight force were acting in the sense of a suspended axis, and were doing so independently of the position of the axis, meaning horizontal, sloping or vertical. The remarks below thus also include such effects as act at least partially like a weight force on the axis.
The disadvantage to this is that a directional detection and monitoring of the torques or the axis forces is now necessary. As a result, common limit values are not able to be used for both directions. The disadvantage is also that the overlaying effect of the weight force means especially that a monitoring of comparatively small amounts of friction-dependent characteristic values is not possible at all or is comparatively imprecise.
Suspended axes are also known from the prior art with elastic weight compensation based on an elastic force. The elastic force can for example be created hydraulically, pneumatically or by means of spring force. However this weight compensation cannot compensate for the effect of the weight force over the entire movement path. As a result, an elastic disruptive force appears in parts of the movement path, which presents itself in such a way that the torque or the force needed increases in a linear fashion with the path, while it decreases in the opposite direction in a linear manner.
The elastic weight compensation makes it possible to compensate to some extent for the mean torque values or force values for both directions of movement. However the minimum and maximum values are determined by the size of the elastic force not usually running in a linear manner over the path. These characteristic values can disadvantageously not be included for monitoring the ease of movement and the wear of the respective axis of the machine. At the same time a monitoring especially of disruptive forces occurring during operation is barely possible.